从透射仪的光学测量系统出发，理论分析了光学测量系统光源对透射仪探测光束远场光斑特性的影响。结合LED光源的发光原理，研究了LED光源的表面特征，分析了LED光源表面特征与透射仪能见度测量的关系。通过仿真实验证实了光源表面特征对透射仪探测光束准直和能见度测量稳定性产生影响。研究结果表明，LED光源表面特征会影响探测光束远场光斑能量分布，在50 m基线下，使得透射仪探测光束准直中心发生位置偏移25 mm，接收光强最大变化20%，从而影响透射仪发射端对准和能见度测量。通过加扩散片和光阑限制可以明显改善远场光斑特性，远场光斑中心能量分布趋于均匀，在中心区域内接收光强变化在1%以内。

The influence of the source in transmittance meter on the farfield spot of probe beam was analyzed in theory. Combined with the principle of luminescence for LED source,the surface characteristics of LED and the relationship between surface characteristics and measurement of transmittance meter were studied . We proved that the surface characteristics of LED could influence the optical path collimation and the stability of measurement by simulation experiment. The results of the study show that the surface characteristics of LED in transmittance meter can influence the energy distribution of the farfield spot of probe beam and make the center position of probe beam offset 25 mm with the baseline of 50 m, the maximum variation of receiving power is 20%, and hence it can influence the optical path collimation of transmitting terminal and the measurement of the transmittance meter. The energy distribution of the farfield spot can become more uniform and the maximum variation of receiving power is 1% by increasing diffusion chip and aperture.